I have the same issue here in Mexico as electrical power here is very expensive and I enjoyed reading the Australian comments as I am from Brisbane. I have also been frustrated with apparent disregard guests have for proper AC use and also leaving everything on when they leave thinking " I paid for this , so I will use it" attitude. Aside from the environmental concerns, they are indirectly increasing the rental costs for other guests.
One thing I have found seems to work quite well is the asking them to shut everything off at the circuit breaker board that is inside each apartment. I put tape over circuit that needs to be left on for the fridge, and for some reason this is more effective than reminding them to separately turn off the AC , lights, overhead fans. I am unsure as yet if turning off AC units that way can ultimately damage the units, but they seem to be fine at the moment.
The other thing is educating guests regarding the 'DRY' setting. As humidity is the issue, this setting works as a dehumidifier. Supposedly it is more efficient. I personally use it myself quite a lot , and with an overhead fan many guests have been happy to use it in conjunction with the COOL mode.
Regarding guests putting the temp setting down to the lowest 16deg C is annoying and unnecessary. Guests think that the room will cool down quicker if set as low as possible , which is simply untrue. I tell guests that setting the units at 16 deg C will cause them to freeze and will stop working ( or spit water out of the units) due to the seasonal humidity . In the House Manual I have written that "appropriate thermostat control for AC is required" . Then follow it up with an introduction message stating setting the AC units at a temperature less than 20 deg C is not permitted and a recommended setting is 22 to 25 degC and to use the SLEEP setting at night. So far, no guests have complained.
That Nest thermostat system sounds like a good idea that I will look into.